Exterior paint can be nothing more than varnish/urethane with pigments added. If the cardboard is already painted with an exterior paint varnish on top is no better than another coat of paint.

: Hi, I'm competing in a cardboard boat regatta and I bought the Helmsman Spar
: Urethane to apply to the cardboard. Now, the cardboard has already been
: painted so I'd be applying over that surface. Is this the right choice?
: Does anyone have any experience with waterproofing cardboard
: (understandable if not)? Thanks.